Thermal energy storage materials

ABSTRACT

A thermal energy storage material (TESM) system (and associated methods) that reproducibly stores and recovers latent heat comprising i) at least one first metal containing material including at least one first metal compound that includes a nitrate ion, a nitrite ion, or both; ii) at least one second metal containing material including at least one second metal compound; and iii) optionally including water, wherein the water concentration if any is present is less than about 10 wt. %; wherein the TESM has a liquidus temperature, T L , from about 100° C. to about 250° C.; and wherein the TESM exhibits a heat storage density from 300° C. to 80° C. of at least about 1 MJ/l; so that upon being used in a system that generates heat, at least a portion of the heat is captured and stored by the TESM and subsequently released for use, and the system is generally resistant to corrosion at temperatures of about 300° C.

CLAIM OF PRIORITY

The present application claims the benefit of the filing dates of U.S.Provisional Patent Application Ser. Nos. 61/030,755 (filed Feb. 22,2008); 61/061,908 (filed Jun. 16, 2008); 61/074,799 (filed Jun. 23,2008); 61/074,840 (filed Jun. 23, 2008); 61/074,869 (filed Jun. 23,2008); 61/074,889 (filed Jun. 23, 2008); and 61/090,084 (filed Aug. 19,2008), and U.S. patent application Ser. No. ______ entitled “HEATSTORAGE DEVICES” filed on Feb. 20, 2009 (attorney docket number 67170B(1062-091)), the contents of which are hereby incorporated by referencein their entirety.

FIELD OF THE INVENTION

The present invention relates to thermal energy storage materials(TESMs), and in one particular aspect, to improved TESM chemicalcompositions.

BACKGROUND OF THE INVENTION

Thermal energy storage materials (TESMs) are known and have been used inapplications for storing heat for subsequent use. Many TESMs are phasechange materials, meaning they undergo a phase change, typically betweensolid state and liquid state, and can store (or release) a considerableamount of the heat, regarded as latent heat from the phase change. Manyof these phase change materials include mixtures of compounds, such thatthe mixture has a lower liquidus temperature than the pure compounds orelements used in the mixture. See generally, Chapter 3, I. Dincer and M.A. Rosen, Thermal Energy Storage Systems and Applications, John Wiley &Sons, London, 2002.

Attention has been directed toward TESMs for use at temperatures belowabout 85° C. Much of this body of work utilizes mixtures of hydrousmetal salts. For example, U.S. Pat. No. 6,627,106 discloses variousphase change materials comprising ternary mixtures of magnesium nitratehexahydrate with other metal nitrates. These mixtures having phasechanges from about 52° C. to about 69° C., depending on the metalnitrates being combined and on the concentration of each metal salt.U.S. Pat. No. 5,785,884 discloses similar ternary mixtures of magnesiumnitrate hexahydrate with sodium nitrate and potassium nitrate. Thesehydrous mixtures of metal nitrates have solid to liquid phasetransitions between 60° C. and 85° C. U.S. Pat. No. 5,728,316 describesbinary mixtures of magnesium nitrate hexahydrate and lithium nitratewhere the molar ratio of magnesium nitrate hexahydrate to lithiumnitrate is from 86:14 to 81:19 with single melting temperature in therange of 71° C. to 78° C. U.S. Pat. No. 6,083,418 discloses phase changematerials comprising a mixture of two metal nitrates (an alkali metalnitrate and an alkaline earth metal nitrate) with an excess of water,such that the phase change material has a small change in densitybetween the solid phase and the liquid phase. The water concentrationranges from 27.9% to 37.2% by weight of the phase change material, withthe specific concentration range of the water dependent on the metalsalts being mixed. U.S. Pat. No. 5,348,080 shows mixtures of water,sodium nitrate, and potassium nitrate and describes phase changematerials having a solid to liquid transition temperature below 0° C.See also, U.S. Pat. No. 5,591,374.

Attention also has been directed toward anhydrous mixtures of metalsalts as phase change materials having very high phase transitiontemperatures. For example, Kerslake, T. W. and M. B. Ibrahim, “Analysisof thermal energy storage material with change-of-phase volumetriceffects,” Journal of Solar Engineering, 115:1, (1993) pp. 22-31,disclose anhydrous mixtures of lithium fluoride and calcium fluoridewhich melt at 1,040 K (767° C.). U.S. Pat. No. 4,657,067 discloses avariety of binary and ternary metal compositions which can be used asthermal energy storage materials. These mixtures all have melting orliquidus temperatures above 500° C. Other phase change materials for usein various applications are disclosed in U.S. Pat. Nos. 4,421,661 and5,613,578. In a paper presented to The Modelica Association, entitled“Analysis of steam storage systems using Modelica” (Modelica 2006,September 4^(th)-5^(th)), Buschle, et al attempt to model steam storagesystems that use unspecified eutectic mixtures of “salts such as:lithium nitrate (LiNO₃), lithium chloride (LiCl), potassium nitrate(KNO₃), potassium nitrite (KNO₂), sodium nitrate (NaNO₃), sodium nitrite(NaNO₂) and calcium nitrate (Ca(NO₃)₂).”

Heretofore, efforts to apply TESMs in commercial applications have alsobeen complicated by difficulties in achieving satisfactory performancein service. Though a TESM may be known to have certain attributes toqualify it as a heat storage material, the assembly of such TESMs into afunctionally operative system has been complicated by theunpredictability of the materials and other considerations, such as TESMinteractions in service with other system materials. For example,corrosion resistance has proven to be a complicating factor for somesystems. Many materials known to be phase change materials are alsocorrosive in many environments. It may also prove difficult to predicthow mixtures of such materials will fare in a desired application.Consider also that, typically, the TESMs must be packaged in some devicethat affords efficient heat exchange. Considerations such as thecompatibility of TESMs with these devices potentially can significantlydeteriorate the performance of a particular TESM, even though the TESMis regarded theoretically as a good performing TESM. For example, somewater-containing TESMs have been observed to cause relatively rapidcorrosion of device packaging. Unpredictable kinetics also havecomplicated the adaptation of various TESM material systems, as has beendurability in the face of intensive thermal cycling.

In view of the above, there continues to be a need for new and efficientTESMs specifically for use at one or more temperatures of about 85° C.to 300° C. and exhibiting one or more of a relatively high energystorage density, a relatively high heat of fusion, relatively lowcorrosivity toward common materials of construction, relatively rapidcrystallization kinetics, long cycle life and long calendar life, andotherwise good compatibility with components of systems in which theTESMs are used.

SUMMARY OF THE INVENTION

The present invention meets some or all of the above needs by providingan improved TESM system that reliably and reproducibly stores andrecovers latent heat comprising: a container having a wall surface; anda TESM in at least partial contact with the wall surface, and including:i) at least one first metal containing material including at least onefirst metal compound that includes a nitrate ion, a nitrite ion, orboth; ii) at least one second metal containing material including atleast one second metal compound; and iii) optionally including water,wherein the water concentration if any is present is less than about 10wt. %; wherein the TESM has a liquidus temperature, T_(L), from about100° C. to about 250° C.; and wherein the TESM exhibits a heat storagedensity from 300° C. to 80° C. of at least about 1 MJ/l; so that uponbeing used in a system that generates heat, at least a portion of theheat is captured and stored by the TESM and subsequently released foruse, and wherein the absolute value of the change in mass of the wallsurface in contact with the TESM is less than about 1 g per m² of thewall surface after 45 days exposure to the TESM at 300° C. in an inertatmosphere.

The TESM system may include a TESM according to the present teachings ina container (e.g., a capsule), optionally being substantially free ofany water and/or hydrogen in some other form, which has an interior wallincluding a metal oxide that has substantially no thermodynamic reactionwith the TESM in service conditions. One particularly preferred TESMincludes two metal containing compounds, each including a cationselected from lithium or sodium, and an anion selected from nitrate ornitrite. The TESM may include one or more other agents that improve itsperformance, such as a thermal conductivity promoter, a corrosioninhibitor, a nucleator, or any combination thereof. The TESMs exhibitany of a number of other attractive characteristics. For example, theTESM may have a value of H_(c50)/H_(C1) greater than 70%, where H_(c50)is defined as the quenched heat of crystallization measured bydifferential scanning calorimetry at a cooling rate of about 50° C./minover a temperature range of T_(L)+50° C. to T_(L)−100° C. and H_(C1) isdefined as the slow cooled heat of crystallization measured bydifferential scanning calorimetry at a cooling rate of about 1° C./min.The TESM may have a liquidus temperature which is less than T_(min) −25°C., wherein T_(min) is the lowest melting temperature of any binary saltwhich can be formed by any anion of the TESM and any cation of the TESM.Other beneficial aspects will be gleaned from the teachings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a calculated phase diagram of a lithium fluoride and lithiumnitrate system.

FIG. 2 is an expected experimental phase diagram of a lithium nitrateand lithium hydroxide system.

FIG. 3 illustrates output of a test for measuring the heat storagedensity from 300° C. to 80° C. and the cumulative heat density from 80°C. to 300° C.

FIG. 4 illustrates the concepts of sensible and latent heat of a TESM.

FIG. 5A illustrates a plot of crystallization curves of an illustrativeTESM within the present teaching for its 1 st, 51 st and 101 st cyclesfrom about 20° C. to about 320° C.

FIG. 5B illustrates the variation in the heat storage density from 320°C. to 20° C. for an illustrative TESM as a function of the number ofcycles between 20° C. and 320° C.

FIG. 5C plots cumulative heat as a function of temperature for anillustrative TESM.

FIGS. 6A and 6B illustrate the thermal properties of a TESM before andafter temperature cycles to about 500° C.

FIG. 7A illustrates crystallization curves of an illustrative TESMwithin the present teaching for its 1 st, 51 st and 101 st cycles fromabout 40° C. to about 305° C.

FIG. 7B illustrates a plot of the variation in the heat storage densityfrom 305° C. to 40° C. for an illustrative TESM as a function of thenumber of cycles between 40° C. and 305° C.

FIG. 7C plots cumulative heat as a function of temperature for anillustrative TESM.

FIGS. 8A and 8B illustrate the thermal properties of a TESM before andafter temperature cycles to about 500° C.

FIG. 9A illustrates crystallization curves of an illustrative TESMwithin the present teaching for its 1 st, 51 st and 101 st cycles fromabout 25° C. to about 200° C.

FIG. 9B illustrates a plot of the variation in the heat storage densityfrom 200° C. to 25° C. for an illustrative TESM as a function of thenumber of cycles between 25° C. and 200° C.

FIG. 9C plots cumulative heat as a function of temperature for anillustrative TESM.

FIG. 10 illustrates the hysteresis of the thermal storage density for aTESM material having low hysteresis and a TESM having high hysteresis.

FIGS. 11A, 11B, and 11C illustrates the melting and crystallizationcharacteristics of erythritol during the first, second and third heatingand cooling cycles, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The teachings herein pertain generally to thermal energy storagematerials (TESMs). As will be illustrated, the TESMs offer a number ofvaluable attributes that render them useful in various applications inwhich it is desired to store heat, and later to release the heat after alapse of some amount of time. To facilitate the storage of heat, thepresent TESMs will preferably be phase change materials, in that theyundergo at least one phase change at a temperature below their maximumoperating storage temperature for a given application. Such a phasechange can be a solid-solid phase change, a solid-liquid phase change,or a liquid-gas phase change. One preferred phase change is asolid-liquid phase change, pursuant to which energy equal to the latentheat of fusion is required to melt the material.

Reference herein will be made to solidus temperatures (T_(s)) andliquidus temperatures (T_(L)). The solidus temperature is thetemperature below which a given material, at thermodynamic equilibrium,is completely solid. The liquidus temperature, as used herein, is thetemperature above which a given material, at thermodynamic equilibrium,is completely liquid. For some materials (e.g. certain pure metals andeutectic compositions at their eutectic point), the liquidus and solidustemperature are one and the same, and are referred to generally as themelting temperature (or the eutectic temperature (T_(e))). Suchtemperature is, theoretically, the temperature above which the materialis liquid and below which the material is solid. As used herein, meltingtemperatures, liquidus temperatures or solidus temperatures (unlessexpressed otherwise) are at approximately one (1) atmosphere pressure.

Metal containing compounds which are mixtures of metals or mixtures ofmetal salts may have a eutectic composition. When the mixture has acomposition which is not at a eutectic composition, it will typicallyundergo a phase transition over a range of temperatures. For example,the mixture may start to melt at the solidus temperature, T_(s) whichmay be the eutectic temperature. As the temperature increases, themixture will continue to melt until it reaches the liquidus temperature,T_(L). Above the liquidus temperature the mixture is completely in theliquid (molten) state.

It is known in the art that the solidus and liquidus temperatures can bedetermined by measurements using differential scanning calorimetry. Forexample, the present teachings contemplate that they are determined bydifferential scanning calorimetry by heating a 3.0-4.0 mg sample fromroom temperature at a rate of about 10° C./min. For example, thesevalues can be realized by use of a differential scanning calorimeter(e.g., a Q2000 differential scanning calorimeter (TA Instruments, NewCastle, Del.)) at a scanning rate of about 10° C./minute, heated to atemperature beyond which no liquid remains in the sample (e.g., to about400° C.), and plotting heat flux against temperature, establishing abase line value, and identifying deviations from the baseline. Astemperature is increased, the start of the deviation corresponds withthe solidus, and the end of the deviation corresponds with the liquidus.

A calculated equilibrium phase diagram (at 1 atm) of lithium fluorideand lithium nitrate (given in FIG. 1) can be used to illustrate theseconcepts. The solidus line for this composition, shown by line S, is ahorizontal line and gives the predicted solidus temperature. One of theliquidus lines is shown by line L. This liquidus line corresponds to theconcentration dependent liquidus temperature of the compositionscontaining less than about 95.3 mole % lithium nitrate. Lithium fluorideand lithium nitrate are seen to have a eutectic composition, shown bypoint E, of about 95.3 mole % lithium nitrate and about 4.7 mole %lithium fluoride, with a eutectic temperature of about 250° C., which isbelow the melting temperature of either pure lithium nitrate or purelithium fluoride. When material having the eutectic composition isheated from room temperature, it will start as a solid phase and undergoa phase transition at about 250° C. where the material will completelymelt at that temperature. This theoretical phase diagram can be furtherunderstood by points P1, P2, P3, P4, and P5, which all correspond to acomposition of about 80 mole % lithium nitrate. Point P1 is locatedbelow the solidus line and shows a condition where, at equilibrium, thematerial is expected to exist only in the solid state. Point P2 is onthe solidus line. Point P3 is between the solidus line and the liquidusline and shows a condition where, at equilibrium, the material isexpected to have both a liquid phase and a solid phase. Point P4 is onthe liquidus line. Point P5 is above the liquidus line and shows acondition where, at equilibrium, the material is expected to exist onlyin the liquid phase.

A calculated phase diagram of lithium nitrate and lithium hydroxide(given in FIG. 2) further illustrates these concepts. These salts have aeutectic composition of about 0.393 mole fraction lithium hydroxide, anda eutectic temperature of about 186° C. which is more than 50° C. lowerthan the melting temperature of pure lithium nitrate. As illustrated amixture of salts may have a large range of compositions which have aliquidus temperature lower than (e.g., at least 25° C., or even at least50° C.) one or all of the pure salts in the mixture.

As will be seen the present invention contemplates that the TESM include(or may even consist essentially of or consist of) at least one firstmetal containing material, and more preferably a combination of the atleast one first metal containing material and at least one second metalcontaining material. The first metal containing material, the secondmetal containing material, or both, may be a substantially pure metal,an alloy such as one including a substantially pure metal and one ormore additional alloying ingredients (e.g., one or more other metals),an intermetallic, a metal compound (e.g., a salt, an oxide orotherwise), or any combination thereof. One preferred approach is toemploy one or more metal containing materials as part of a metalcompound; a more preferred approach is to employ a mixture of at leasttwo metal compounds. By way of example, a suitable metal compound may beselected from oxides, hydroxides, compounds including nitrogen andoxygen (e.g., nitrates, nitrites or both), halides, or any combinationthereof. One particularly preferred metal compound includes at least onenitrate compound, at least one nitrite compound or a combinationthereof. Preferred TESMs for use herein thus may be part of a binarycomponent material system, pursuant to which two discrete compounds arecombined with each other (e.g., a first metal nitrate or nitritecombined with second metal nitrate or nitrite). It is possible thatternary, quaternary or other multiple component material systems may beemployed also. The TESMs herein may be mixtures of two or more materialsthat exhibit a eutectic.

The metals for use herein may be any suitable metal capable of beingused in a material system that has a liquidus temperature from about 85°C. to about 350° C. Examples of suitable metals may be selected fromtin, lead, zinc, magnesium, aluminum, lithium, sodium, potassium,beryllium, calcium, gallium, indium, thallium, strontium, barium,rubidium, francium, bismuth, cadmium, antimony, tellurium, or anycombination thereof. Still more preferred metals are selected fromsodium, potassium, lithium, magnesium, or any combination thereof. Evenstill more preferred metals are sodium, lithium, potassium, or anycombination thereof. Most preferred are sodium, lithium or a combinationthereof. It will be appreciated from the above, that certain of thementioned metals have melting points outside of the range of about 85°C. to about 350° C.; such a metal is contemplated to be employed otherthan in its pure form.

The liquidus temperature of the TESMs may be low relative to the meltingtemperatures of the individual components in the TESMs. As such, theTESMs may have a liquidus temperature less than about T_(min) −25° C.,where T_(min) is the lowest melting temperature of any salt (e.g., abinary compound salt) which can be formed by a significant anion (e.g.,an anion present at a concentration of at least about 3 mole %, based onthe total moles of anions in the TESM) and a significant cation (e.g., acation present at a concentration of at least about 3 mole %, based onthe total moles of cations in the TESM) of the TESMs. The TESMs may havea liquidus temperature which more preferably is less than about T_(min)−40° C., and most preferably is less than about T_(min) −50° C. Theliquidus temperature of the TESMs may also be characterized in relationto the minimum liquidus temperature of all the composition comprisingthe anions and cations (e.g., comprising the significant anions andcations) in the TESMs, T_(liquidus,min). In certain mixtures,T_(liquidus,min) may be the eutectic temperature. The TESMs may have aliquidus temperature less than about T_(liquidus,min) +50° C., morepreferably less than about T_(liquidus,min) +25° C., and most preferablyless than about T_(liquidus,min) +10° C.

Thermal energy storage materials which are useful for the presentinvention may have a liquidus temperature from about 85° C. to about350° C., such as about 95° C. to about 300° C. (e.g., from about 100° C.to about 200° C.). The liquidus temperature is preferably less thanabout 350° C., more preferably less than about 310° C. (e.g. less thanabout 300° C., or even 270° C.). Thermal energy storage materials havinga liquidus temperature less than about 250° C. are also contemplated.The liquidus temperature is preferably greater than about 95° C., morepreferably greater than about 100° C. (e.g. greater than about 125° C.,or even 175° C.). Higher liquidus temperatures may be possible also(e.g., to about 700° C.).

To achieve this, it may be possible to employ one or more materials thathave a liquidus temperature that exceeds about 200° C. One or more ofthe metal containing materials may have a liquidus temperature less thanabout 450° C. and, more preferably, less than about 400° C., or evenless than about 350° C. It is possible that a composition herein mayinclude at least a first metal compound and a second metal compound. Atleast one of the compounds (e.g., the first metal compound) may have aliquidus temperature, T_(L) less than about 450° C. and more preferably,less than about 400° C., still more preferably less than about 350° C.(e.g. less than about 310° C.). The first metal compound may also have aliquidus temperature higher than about 85° C., and more preferably,higher than about 90° C., still more preferably higher than about 100°C., (e.g. higher than about 125° C., or even 175° C.).

For systems that include two or more metal containing compounds, one ofthe metal containing compounds (e.g., the second) preferably has aliquidus temperature higher than the liquidus temperature of the other(e.g., the first) metal containing compound. One (e.g., the second)metal containing compound may have a larger entropy of fusion densitythan the other (e.g., the first) metal containing compound. By way ofexample, the second metal containing compound may have a liquidustemperature, T_(L), greater than about 310° C. and more preferably,greater than about 400° C., (e.g. greater than about 440° C.).

If the TESM is a mixture that is not at its eutectic composition (e.g.,it is a binary composition that may have a eutectic, but it is not usedat its eutectic composition), the difference between the liquidustemperature and the solidus temperature is preferably less than about150° C., more preferably less than about 100° C., and most preferablyless than about 50° C. By way of example, without limitation, solidustemperatures may range from about 85° C. to about 350° C. The solidustemperature may be less than about 300° C., more preferably less thanabout 270° C. (e.g. less than about 250° C.). The solidus temperaturemay be greater than about 100° C., more preferably greater than about125° C. (e.g. greater than about 175° C.). For example, the solidustemperature may be from about 100° C. to about 300° C.

Preferably, the TESMs systems employing them herein will be essentiallyfree of water, whether chemical water (e.g., chemically bound water),physical water (e.g., physically bound water) or both (i.e., it will besubstantially anhydrous). Thus, the TESM or system that employs the TESMmay have a concentration of water, measured for example by Karl Fischertitration (e.g., using a Sartorius WDS 400) less than about 5000 ppm,2500 ppm, 1000 ppm, 500 ppm, 400 ppm, 350 ppm, 250 ppm, 100 ppm, 50 ppm,25 ppm, 10 ppm, or even below the detection limit of typical Karl Fishertitration techniques using a sample size of about 1 gram.

It is possible, however, that some water optionally may be employed inTESMs of the present invention, such as water as a result of hydrationof a metal salt, by the addition of water as a diluent, or otherwise.For example, water may be present in an amount less than about 10percent by weight of the total material in the thermal energy storagematerial system, more preferably less than about 5 percent by weight ofthe total material in the thermal energy storage material system, andstill more preferably less than about 2 percent by weight of the totalmaterial in the thermal energy storage material system, and even stillmore preferably less than about 1 percent by weight of the totalmaterial in the thermal energy storage material system (e.g., less thanabout 0.5 percent by weight of the total material in the thermal energystorage material system).

TESMs herein will exhibit attractive performance characteristics thatallow it to store large quantities in a given volume of material. Such amaterial thus may be characterized as having a relatively high “latentheat of fusion density”, which is also referred to as “heat of fusiondensity”. The latent heat of fusion density, which is the product of thelatent heat of fusion (MJ/kg) (measured using differential scanningcalorimetry, e.g., using a TA Instruments DSC Q2000 calibrated for heatcapacity with a sample size of about 10 mg and a cooling rate of about10° C./min, and determined by integrating the maxima of the plot of heatflow against temperature) and the density at 25° C. (kg/l) (and whichreflects a value associated with the release of heat during a phasetransition from a liquid to a solid state), may be greater than about0.35 MJ/l; preferably greater than about 0.45 MJ/l, more preferablygreater than about 0.65 MJ/l, more preferably greater than about 0.8MJ/l. more preferably greater than about 1 MJ/l, still preferablygreater than about 1.2 MJ/l, even still more preferably greater thanabout 1.4 MJ/l and most preferably greater than about 1.6 MJ/l (e.g.,greater than about 2.0 MJ/l).

Another measure of the capacity to store thermal energy may be the “heatstorage density” (HSD_(T2,T1)) given by the amount of the heat releasedfrom 1 liter (at 25° C. and 1 atmosphere pressure) of a thermal energystorage material as it is cooled from a high temperature T₂ to a lowtemperature T₁ (where T₂ and T₁ are both in ° C.). The HSD_(T2,T1) maybe measured by differential scanning calorimetry calibrated for heatcapacity at a constant heating rate of about 10° C./min and a constantcooling rate of about −10° C. as illustrated in FIG. 3. Morespecifically, the heat storage density (HSD_(T2,T1)) of the thermalenergy storage material may be determined by first measuring the roomtemperature (23° C.) density of the material in units of kilograms perliter (kg/l). Next, a 3.0-4.0 mg sample of the thermal energy storagematerial is heated at 10° C./min from room temperature to T₂+50° C. in aQ2000 differential scanning calorimeter (TA Instruments, New Castle,Del.). The sample is then cooled at 10° C./min back to room temperature.The energy released per kg of the thermal energy storage material, whenthe material is cooled from T₂ to T₁ is measured by the differentialscanning calorimeter in units of J/kg. The heat storage density(HSD_(T2,T1)), in units of J/l, is then calculated by multiplying thedensity by the energy released upon cooling the material.

As seen in FIG. 3, by way of illustration, using T₂=300° C. and T₁=80°C., the released heat 150 (which is represented in this plot of the heatflow as a function of temperature as the area under the cooling curvebetween 300° C. and 80° C. multiplied by the density of the TESM wherethe density may be measured at about 25° C.) may thus include what theskilled artisan would recognize as sensible heat 151 and latent heat152. FIG. 3 also illustrates the cumulative heat density between 80° C.and 300° C. 153 which is the heat required to heat one liter of thematerial from 80° C. to 300° C. For example, the heat storage densityfrom 300° C. to 80° C. (HSD_(300,80)) would be regarded as the amount ofheat released from 1 liter of a thermal energy storage material as it iscooled from 300° C. down to 80° C. HSD_(300,80) may be greater thanabout 1.0 MJ/l. HSD_(300,80) is preferably greater than about 1.2 MJ/l,more preferably greater than about 1.4 MJ/l and most preferably greaterthan about 1.6 MJ/l. Similarly, HSD_(350,80) would be regarded as theamount of heat released from 1 liter of a thermal energy storagematerial as it is cooled from 350° C. down to 80° C. HSD_(350,80) may begreater than about 1.0 MJ/l. HSD_(350,80) is preferably greater thanabout 1.2 MJ/l, more preferably greater than about 1.4 MJ/l and mostpreferably greater than about 1.6 MJ/l. The latent heat and the sensibleheat are further illustrated in FIG. 4 where the heat (e.g., the storedheat) is plotted against the temperatures. As illustrated in FIG. 4 theTESM (or phase change material) may have a generally constant heatcapacity, c_(p), in the solid state, or over a range of temperatures inthe solid state, and the sensible heat 151 increases with thetemperature of the TESM (e.g. at a constant rate in the solid stateand/or at a constant rate in the liquid state), whereas the latent heat152 may occur at a very narrow temperature range (e.g. at a singletemperature).

Another characteristic of TESM materials is that they may release alarge amount of heat when cooled from about 200° C. (preferably from atleast a partially liquid state) to about 50° C. (or even from about 150°C. to about 50° C.) while occupying a small volume. As such, the TESMmay be characterized by a heat storage density from 200° C. to about 50°C. (i.e. a HSD_(200,50)) that may be greater than about 0.5 MJ/l,preferably greater than about 0.6 MJ/l, more preferably greater thanabout 0.75 MJ/liter and most preferably greater than about 0.9 MJ/liter.The HSD_(150,50) of the TESM may be greater than about 0.45 MJ/l,preferably greater than about 0.5 MJ/l, more preferably greater thanabout 0.6 MJ/liter and most preferably greater than about 0.75 MJ/liter.

Yet another characteristic of TESMs in accordance with the presentteachings is that they exhibit a relatively rapid rate ofcrystallization, such that the TESM is free of supercooling or has arelatively low amount of supercooling when the TESM is quickly cooled.For example, the TESM (e.g., a 10 mg sample of the TESM) may have avalue of H_(C50)/H_(C1) greater than 0.70, preferably greater than 0.80,and more preferably greater than 0.90, where H_(C50) is defined as thequenched heat of crystallization measured by differential scanningcalorimetry at a cooling rate of about 50° C./min over a temperaturerange of T_(L)+50° C. to T_(L)−100° C. and H_(C1) is defined as the slowcooled heat of crystallization measured by differential scanningcalorimetry at a cooling rate of about 1° C./min.

In certain applications, such as transportation, it may be desirable forthe thermal energy storage material to have a relatively low density,e.g. below about 3.6 kg/l, or preferably, below about 3.3 kg/l, and morepreferably, below about 3.0 kg/l and most preferably below about 2.5kg/l (e.g., below about 2.3 kg/l). The density (kg/l) of the thermalenergy storage material is measured at room temperature (25° C.)according to ASTM D854. The TESMs likewise may have a relatively lowspecific gravity as measured for example by a gas pycnometer. Forexample, the specific gravity of the TESM most preferably may be lessthan about 2.5 kg/l (e.g., less than about 2.3 kg/l).

Another possible characteristic of TESMs in accordance with the presentteachings may be that they will be substantially free of a liquid to gasphase transition or a solid to gas phase transition at a temperaturebelow about 250° C. and more preferably at a temperature below about300° C. The TESM may also be stable in that it does not decompose at atemperature below about 250°, more preferably at a temperature belowabout 300° C., and most preferably at a temperature below about 350° C.Preferably, the total equilibrium vapor pressure of the TESM is lessthan 1 atmosphere at about 250° C., preferably at about 300° C., andmore preferably at about 350° C. Because it is possible that manyorganic compounds may decompose at relatively low temperatures, it maybe desirable for the concentration (in wt % based on the total weight ofthe TESM) of organic compounds in the TESM, if any are present, to beless than about 5 wt %, more preferably less than about 1 wt %. Morepreferably the TESM may be essentially free of organic compounds or evenfree of organic compounds. For example, the TESM may be free of anamount of organic compound detectable by FTIR (e.g., the FTIR spectrumof the TESM may have no detectable peaks corresponding to acarbon-hydrogen bond). In this regard, the TESMs may be free of waxes,the TESMs may be free of alcohols (e.g., polyhydric alcohols) or both.The TESMs may be free of a magnesium compound, such as a magnesiumnitrate hexahydrate.

Of course, the TESMs may be free of any detectable hydrogen atoms. Theymay thus also be free of —OH anions. However, as seen from the teachingsabove, it may be possible to optionally use small concentrations ofhydrogen (e.g. less than about 10 mole % hydrogen, less than about 5mole % hydrogen, or even less than about 2 mole % hydrogen based on thetotal number of moles of atoms in the TESM) while maintaining one ormore feature of the TESM within the teachings.

Another characteristic of preferred compositions herein is their abilityto withstand repeated thermal cycling and retain one or more (andpreferably all) of the above characteristics. For instance, TESMs inaccordance with the invention may possess the characteristics that, uponrepeatedly cycling between the melt and the solid state there isrelatively insignificant deterioration in the ability of the TESM tostore and release thermal energy. For example a TESM may initially becharacterized by an initial heat storage density between T₂ and T₁(HSD_(T2,T1,0)), and after being cycled between T₁ and T₂ for n cycles(e.g., about 100 cycles, more preferably at least 1000 cycles, and stillmore preferably at least about 5000 cycles) at a heating rate of 10°C./min and a cooling rate of −10° C./min, it may be characterized by anaged heat storage density between T₂ and T₁ (HSD_(T2,T1,n)). Thustested, a suitable TESM may be characterized by a ratio of the agedHSD_(320,20,100) to the initial HSD_(320,20,0) which is greater thanabout 0.6, preferably greater than about 0.8, more preferably greaterthan about 0.9, and most preferably greater than about 0.95. SuitableTESM may also be characterized by a ratio of the aged HSD_(305,40,100)to the initial HSD_(305,40,0) which is greater than about 0.6,preferably greater than about 0.8, more preferably greater than about0.9, and most preferably greater than about 0.95. Suitable TESM may alsobe characterized by a ratio of the cycled/aged HSD_(T2,T1,n) to theinitial HSD_(T2,T1,0) which may be greater than about 0.6, preferablygreater than about 0.8, more preferably greater than about 0.9, and mostpreferably greater than about 0.95 and where T₂ may be about 5° C. toabout 100° C. above the liquidus temperature of the TESM, T₁ may be fromabout 0° C. to about 80° C., and n may be at least about 100 cycles(e.g. 100 cycles), more preferably at least about 1000 cycles (e.g.,1000 cycles), and more preferably at least about 5000 cycles (e.g. 5000cycles). For example, a TESM having a liquidus temperature of about 120°C. to about 180° C. may be characterized by a ratio of the agedHSD_(200,25,100) to the initial HSD_(305,25,0) which is greater thanabout 0.6, preferably greater than about 0.8, more preferably greaterthan about 0.9, and most preferably greater than about 0.95.

In one particularly preferred aspect, the improved thermal energystorage material (and any system incorporating it) is particularlycharacterized as including a substantially anhydrous mixture of a) atleast one first metal containing compound that includes a nitrate ion, anitrite ion or both; and b) a second metal compound (e.g., another metalcontaining compound that includes a nitrate ion, a nitrite ion or both).Preferably, either or both of the first or second metal containingcompound includes, consists essentially of, or consists of at least onemetal salt, and more preferably includes, consists essentially of, orconsists of a metal nitrate or a mixture of metal nitrates, a metalnitrite or a mixture of metal nitrites, or any combination thereof.Exemplary metal nitrates may include sodium nitrate, lithium nitrate, orany combination thereof. Exemplary metal nitrites may include sodiumnitrite, lithium nitrite, or any combination thereof.

Without limitation, the TESMs may include a mixture of lithium nitrateand one or more of lithium nitrite, sodium nitrate or sodium nitrite.For example the TESM may include lithium nitrate at a concentrationgreater than about 35 mole %, preferably greater than about 40 mole %,and more preferably greater than about 44 mole %. The concentration ofthe lithium nitrate may be less than about 65 mole %, preferably lessthan about 60 mole %, and most preferably less than about 54 mole %. Oneparticular preferred TESM has a mixture of lithium nitrate and sodiumnitrate. The lithium nitrate may be present in the above concentration,and the TESM may include sodium nitrate at a concentration greater thanabout 35 mole %, preferably greater than about 40 mole %, and morepreferably greater than about 46 mole %. The concentration of the sodiumnitrate may be less than about 65 mole %, preferably less than about 60mole %, and most preferably less than about 56 mole %. Another possiblecombination may substitute lithium nitrite or sodium nitrite for one oflithium nitrate or sodium nitrate per the above teachings.

There may be greater than about 30 mole % Li cations based on the totalmoles of metal cations in the TESM, and greater than about 30 mole %nitrate anions based on the total moles of anions in the TESM. It ispossible that the TESM may have greater than about 15 mole % nitriteanions based on the total moles of anions in the thermal energy storagematerial. The thermal energy storage material may have from about 40 toabout 80 mole % (e.g. from about 55 to about 75 mole %) Li cations basedon the total moles of metal cations in the thermal energy storagematerial, b) from about 40 to about 80 mole % (e.g. from about 55 toabout 75 mole %) nitrate anions based on the total moles of anions inthe thermal energy storage material, and c) from about 20 to about 60mole % (e.g. from about 25 to about 45 mole %) nitrite anions based onthe total moles of anions in the thermal energy storage material. TheTESM may have greater than about 2 mole % (e.g. greater than about 5mole %, or greater than about 10 mole %) Na cations based on the totalconcentration of metal cations in the thermal energy storage material.

Though sodium and lithium are most preferred as cations of the compoundsused in TESMs herein, other metals may also be substituted (instead oflithium or sodium) in the above amounts. Moreover, other anions may besubstituted in the above amounts. Suitable cations may include at leastone metal cation as taught previously for preferred metals. For example,the at least one metal cation may be selected from the group consistingof Li, Na, K, Be, Mg, Ca, Al, and Ga). The anions may be a monoatomicanion or a polyatomic anion. Examples of monoatomic anions suitable forthe first metal salt include halide ions (e.g. chloride, fluoride,bromide, and iodide ions). Examples of polyatomic anions include nitrate(NO₃ ⁻), nitrite (NO₂ ⁻), and phosphate (PO₄ ³⁻) ions. Additionalexamples of polyatomic anions include acetate (CH₃COO⁻), carbonate (CO₃²⁻), sulfate (SO₄ ²⁻), and hydroxide (OH⁻) ions. Suitable anions mayinclude, without limitation, anions which contain at least one atomselected form the group consisting of S, N, F, Cl, Br, I, P, B, and C. Apreferred class of the metal containing compound may preferably containat least one anion that contains a N atom (e.g. two or more anions whichcontain a N atom). For example, the preferred class of the metalcontaining compound may contain a nitrite ion, a nitrate ion, or both.

One possible approach is to combine lithium nitrate or lithium nitritewith one or more second or additional compound, such as lithiumfluoride, lithium chloride, sodium hydroxide, sodium fluoride, sodiumchloride, magnesium hydroxide, magnesium fluoride, magnesium chloride,or any combination thereof. Such second or additional compounds (e.g. athird metal, compound or third metal salt) may be selected from themetal containing materials previously identified and preferably will bedifferent in composition, property or both from the lithium nitrate orlithium nitrite or other first metal containing material.

Other examples of the second or additional compounds might includepotassium hydroxide, potassium fluoride, potassium chloride, calciumhydroxide, calcium fluoride, and calcium chloride. Metal bromides mayalso be used. The second or additional metal compound may containlithium hydroxide. Examples of second metal containing compounds whichcontain multiple metal ions or multiple anions include: mixtures ofmagnesium fluoride, sodium fluoride and lithium fluoride; mixtures ofsodium chloride, sodium fluoride and lithium fluoride; or mixtures oflithium hydroxide, lithium fluoride, sodium fluoride, and magnesiumfluoride.

As illustrated before (with the sodium and/or lithium nitrates and/ornitrites) a suitable TESM may consist essentially of a binary mixture ofa first metal compound and a second metal compound. As one example, thefirst metal compound may be lithium nitrate and the second metalcompound may be a different compound (e.g., lithium hydroxide). In sucha mixture, the concentration of lithium nitrate is preferably at leastabout 25 mole % and the concentration of the lithium hydroxide ispreferably less than about 75 mole % based on the total concentration oflithium nitrate and lithium hydroxide. As a second example, the firstmetal compound consists essentially of sodium nitrate and the secondmetal compound consists essentially of lithium hydroxide. In anotherexample, the first metal compound consists essentially of lithiumnitrate and the second metal compound consists essentially of lithiumfluoride. In this example, the concentration of the lithium nitrate ispreferably at least about 50 mole % and the concentration of the lithiumfluoride is preferably less than about 50 mole %, based on the totalconcentration of lithium nitrate and lithium fluoride. The mixture mayalso be a binary mixture where the first metal compound consistsessentially of sodium nitrate and the second metal compound consistsessentially of lithium fluoride, preferably where the concentration ofthe sodium nitrate is at least about 50 mole % and the concentration ofthe lithium fluoride is less than about 50 mole %, based on the totalconcentration of sodium nitrate and lithium fluoride.

A suitable thermal energy storage material may also be a mixture of morethan two compounds (e.g. at least three compounds, such as three metalcompounds). For example, the thermal energy storage material may includeconsist essentially of sodium nitrate, lithium nitrate and lithiumfluoride. Here, it is preferred that the combined concentration of thesodium nitrate and lithium nitrate is at least about 50 mole % (e.g., atleast about 75 mole %, or even at least about 95 mole %) based on thetotal concentration.

More generally, the thermal energy storage material may include orconsist essentially of a first metal compound which is lithium nitrate,sodium nitrate, or a combination thereof, and one or more additionalmetal compounds which may be selected from the group consisting ofsodium hydroxide, sodium chloride, sodium fluoride, magnesium hydroxide,magnesium, magnesium fluoride, lithium hydroxide, lithium chloride, andlithium fluoride. In one example, the second metal compound consists ofone, two, or three metal compounds selected from the group consisting ofmagnesium fluoride, sodium fluoride, and lithium fluoride and theconcentration of the first metal compound is from about 50 to about 95mole %, based on the total concentration of the first metal compound andthe second metal compound. One or more additional metal compounds may beselected from the group consisting of sodium chloride, sodium fluoride,and lithium fluoride and the concentration of the first metal compoundis from about 50 to about 95 mole %, based on the total concentration ofthe first metal compound and any additional metal compound. For examplethe first metal compound may consist essentially of lithium nitrate,sodium nitrate, or a combination thereof, and the additional metalcompound may consist essentially of sodium fluoride and lithiumfluoride. If present the concentration of magnesium fluoride should beless than about 30 mole %, based on the total concentration.

One TESM which may be used includes Li cations, nitrate and/or nitriteanions, and at least one additional anion or cation. For example, theconcentration of the Li cations may be greater than 30 mole % based onthe total moles of metal cations in the TESM, the concentration of thenitrate anions may be at least 30 mole % based on the total moles ofanions in the TESM, or both. Such a TESM may also include nitrite anionsat a concentration greater than about 15 mole % based on the total molesof anions in the TESM. For example, the TESM may contain a) from about40 mole % to about 80 mole % Li cations (more preferably from about 55to about 75 mole % Li cations) based on the total concentration ofcations in the TESM; b) from about 40 mole % to about 80 mole % nitrateanions (more preferably from about 55 to about 75 mole % nitrate anions)based on the total concentration of anions in the TESM; and c) fromabout 20 mole % to about 60 mole % nitrite anions (more preferably fromabout 25 to about 45 mole % nitrate anions) based on the totalconcentration of anions in the TESM. Any of the above TESMs mayadditionally include Na cations, K cations, or both. For example, theadditional Na and/or K cations may be present at a concentration of atleast about 2 mole %, preferably at least about 5 mole % and mostpreferably at least about 10 mole %. It is further contemplated that Mgcations, Ca cations, or both may be added to any of the abovecompositions.

In addition to the anions and cations, the TESM compositions or systemsherein may also include one or more other agents or ingredients as thefollowing illustrates. For example, preferred compositions or systemsherein may include the TESMs above described and one or more agentsselected from a thermal conductivity promoter, a corrosion inhibitor, anucleator, or any combination thereof.

A suitable TESM may have a high thermal conductivity in order that theheat being transferred to the TESM is relatively rapidly conductedthroughout the material. If the metal compound or the mixture of metalcompounds used in the TESM has a low thermal conductivity, an additionalcompound or ingredient can be added as a thermal conductivity promoter,thereby increasing the effective thermal conductivity of the material.Any suitable art-disclosed conductivity promoter suitable for theoperating conditions could be utilized. Examples of conductivitypromoters include, without limitation, metal whiskers, metal platelets,graphite, divided graphite, graphene, or any combination thereof.Similarly a conductive structure such as a wire mesh (e.g. a copper wiremesh) or metal wool could be incorporated into the thermal energystorage material. Preferably, the thermal conductivity promoter has athermal conductivity of at least about 100 W/(m·K), more preferably atleast about 220 W/(m·K) (e.g. at least about 300 W/(m·K)) as measuredfor example by ASTM Standard E1225-04. In one embodiment of theinvention, the conductivity promoter remains in the solid state at themaximum operating storage temperature.

In general, the TESM systems herein will include a TESM that iscompatible with any material it contacts in service. As will bediscussed, it is contemplated that systems herein will employ acontainer (e.g., capsules) that contain a selected volume of the TESM.It is contemplated that the container may be sealed. It is alsocontemplated that the container may include an inner wall surface thatcontacts the TESM, which has a metal oxide, complex oxide, or otherstable metal compound on it. For example, the container may be metal,and may have a native oxide on it. Systems herein are contemplated toexhibit compatibility between any metal, metal oxide and TESM in contactwith each other over the operational temperature range of the system. Tothis end, the system is generally resistant to corrosion mediated by theinteraction between the TESM and the container wall surface,particularly after prolonged exposure (e.g., 45 days or longer) to anelevated temperature (e.g., of about 300° C.). Corrosion resistance ofthe container of the system may be determined by placing a sample of thecontainer material having a total surface area of about 60 cm² into acrucible made of the same material as the sample and filled with moltenTESM. The surfaces of the sample are contacted with the TESM throughoutthe test, and the crucible is heated in an autoclave purged and filledwith an inert gas such as dry nitrogen at about 300° C. for about 45days. Thus tested, the sample of the container material upon extractionfrom the crucible and removal of TESM residue preferably exhibits anabsolute value of weight change (e.g., a weight increase due tooxidation, or a weight decrease due to etching) of less than about 6 mg,more preferably less than 3 mg, and most preferably less than about 1mg. The weight change may be expressed in terms of the ratio of theweight gain to the surface area of the sample being tested. For examplethe weight gain per surface area may be less than 1 g/m², preferablyless than 0.5 g/m², more preferably less than 0.17 g/m² and mostpreferably less than 0.1 g/m². Such a material would thus be generallycorrosion resistant consistent with the present teachings. A stillpreferred characteristic of the generally corrosion resistant materialsherein is that they will also exhibit the above characteristics afterrepeated (e.g., 100 or more cycles, 1000 or more cycles, or even 5000 ormore cycles) cycling from room temperature to about 300° C. (e.g., asheated and cooled at a rate of about 10° C./min.

TESMs (though they may be intrinsically a corrosive material) suitablymay be employed so it will not significantly corrode any container(e.g., any capsule as will be described) in which it is stored.Accordingly, optionally, an agent for inhibiting corrosion may beemployed, in an amount sufficient for resisting corrosion, as part of aTESM or system herein. Such an agent may be a thermodynamic corrosioninhibitor, a kinetic corrosion inhibitor, or both. The agent may includea metal oxide, a precursor that converts to a metal oxide, or anycombination of such materials. The agent may be such that it can phaseseparate from the TESM and deposit onto a wall of a container that holdsthe TESMs (e.g., a wall of a capsule of a blister pack). Therefore, itmay be advantageous to add an excess of a metal oxide to the thermalenergy storage material composition such that the solubility limit isreached or exceeded. As an alternative, a compound which decomposes intoa metal oxide could be included in the TESM. As an example, aluminumnitrate readily decomposes into aluminum oxide, which may therefore bean effective agent.

Some TESMs, when placed in a container that has a metal oxide surface,e.g. a native metal oxide layer, will dissolve the metal oxide layeruntil a saturation level of the metal oxide is reached. If the volume ofthe thermal energy storage material is high enough, it may even dissolvethe entire metal oxide layer. Once the metal oxide layer is partially orcompletely removed, the container may be more susceptible to corrosiveattack, perhaps by the thermal energy storage material. If an excess ofmetal oxide is added to the thermal energy storage material (e.g. anamount over the saturation limit of the metal oxide in the thermalenergy storage material), the metal oxide layer on the surface of thecontainer will be stable and will not dissolve over time. To help assureeffectiveness, preferably the agent (e.g. a metal oxide) will be presentin a concentration above the saturation concentration of the agent (ametal oxide) in the thermal energy storage material. Typically theconcentration of the metal oxide in the TESM is less than about 20 wt %(preferably less than 5 wt. %) by weight of the thermal energy storagematerial. The concentration of the metal oxide in the TESM should be atleast 0.5% by weight, and more preferably, at least 2% by weight of theTESM. It should be recognized that this agent and other potentialadditives, may not necessarily be in a solution with the TESM, orotherwise might not be regarded as being in the TESM. If the latter isthe case, then the concentrations recited herein for such agent oradditive, refers to the percentage by weight that the agent or additiveoccupies relative to the overall system of materials (i.e., thecombination of TESM and any agent or additive) that includes the agentor additive and the TESM. As an example, aluminum nitrate may be aprecursor to aluminum oxide and may be used as such an agent. One sucheffective agent may be alumina (Al₂O₃) powder at a concentration ofabout 2 mole %, based on the total concentration of the TESM.

It is also contemplated that the TESMs of the present invention mayinclude one or more agents for assisting crystallization of the TESMsduring cooling of the TESMs. The TESM may thus also include a nucleatoror other means for increasing the crystallization rate, increasing thecrystal nucleation rate, increasing the crystal growth rate, increasingthe number of crystals, or any combination thereof. For instance, it ispossible that the TESMs may include a suitable nucleator, or anotheragent for providing a relatively low energy site from which crystals arenucleated, grown or both during cooling. By way of example, it may bepossible to include as part of the TESMs composition up to about 10percent by weight of such an agent (e.g., a nucleator), and morepreferably less than about 5 percent by weight, and still morepreferably less than about 1 percent by weight (e.g., about 0.5 percentby weight or less). Without limitation, examples of a suitable nucleatormay include, a compound including an alkaline metal, a metal oxide, acarbonate (e.g. an alkaline metal carbonate), a sulfate, a phosphate, afluoride, a borate or any combination thereof. Suitable nucleators arepreferably in a crystalline phase a temperature greater than theliquidus temperature of the TESM.

The present invention is also directed at a process for preparing TESMssuch as those described herein. In many cases, these compounds can beeasily combined by heating the compounds to a temperature greater thanthe liquidus temperature of the metal compound having the highestliquidus temperature and mixing. However, in some mixtures, one of themetal compounds may have a decomposition temperature lower than theliquidus temperature of the metal compound having the highest liquidustemperature. In such a situation, another mixing process may beemployed. For example, it is contemplated that one process for mixingthe thermal energy storage material may include selecting a combinationof materials to be employed in a material system, the material havingcompounds with different liquidus temperatures. The highest liquidustemperature metal compound having a first liquidus temperature is heatedto a temperature greater than the first liquidus temperature (preferablyin the absence of any of the other materials to be employed in thesystem). A decomposable metal compound having a second liquidustemperature and a decomposition temperature is mixed with the highestliquidus temperature metal compound. Preferably, the first liquidustemperature is greater than second liquidus temperature and thetemperature of the mixture decreases to a temperature lower than thedecomposition temperature upon addition of the decomposable metalcompound to the molten highest liquidus temperature compound.

In another example, a process for mixing the thermal energy storagematerial may include steps of selecting a combination of at least threematerials (e.g. a decomposable metal compound (which may be the firstmetal compound), a highest liquidus temperature metal compound (whichmay be the second metal compound), and a third metal compound having athird liquidus temperature)) to be employed in a material system, sothat the compounds of the material each have different liquidustemperatures. The highest liquidus temperature metal compound having afirst liquidus temperature and the third metal compound having a thirdliquidus temperature are heated to a first mixing temperature which isgreater than the third liquidus temperature (preferably greater than thefirst liquidus temperature) to form a first mixture having a firstsolidus temperature lower than the first liquidus temperature. Next, thedecomposable metal compound having a second liquidus temperature and adecomposition temperature (which is lower than the first liquidustemperature) and the first mixture are mixed at a second mixingtemperature below the decomposition temperature to form a second mixturehaving a second solidus temperature which is lower than thedecomposition temperature. Preferably, the first solidus temperature islower than the decomposition temperature. The second mixing temperaturemay or may not be lower than the first solidus temperature.

In another example, the improved process can include steps of selectinga combination of at least two materials (including a decomposable metalcompound and a highest liquidus temperature metal compound) to beemployed in a material system, the materials including compounds havingdifferent liquidus temperatures. The highest liquidus temperature metalcompound having a first liquidus temperature and the decomposable metalcompound having a decomposition temperature are provided. The highestliquidus temperature metal compound and the decomposable metal compoundare heated to a temperature below the decomposition temperature to forma first mixture. In this process, there may also be a step ofencapsulating the at least two materials (e.g. the metal compounds) thatare used to make the thermal energy storage material prior to theheating step.

In any of these processes there may also be one or more intermediatesteps of adding one or more additional metal compounds before the stepof adding the decomposable metal compound. It is further contemplatedthat the improved process will include a cooling step before adding thedecomposable metal compound. A step of encapsulating the thermal energystorage material is also contemplated. It may occur before, during orafter the compounds are heated and mixed. A step of adding one or moreagents selected from a thermal conductivity promoter, a corrosioninhibitor, a nucleator, or any combination thereof is also possible.

The process may also include one or more steps of drying the TESM and/orone or more of the materials used for preparing the TESM (e.g., at atime immediately preceding sealing the TESM in a container). Forexample, the process may include a step of exposing one of thesematerials to a dry environment, such as an environment containing adessicant. The process may include a step of removing water by heatingone of the materials (e.g., the TESM), preferably in an inert orrelatively dry atmosphere (e.g., having a relative humidity less thanabout 20%, preferably less than about 10%, more preferably less thanabout 5%, and most preferably less than about 1%. For example, the TESMand/or a material used for preparing the TESM may be heated to atemperature greater than about 100° C., preferably greater than about150° C., more preferably greater than about 200° C., and most preferablygreater than about 250° C. The heating time may be long enough to removeessentially all of the water or to reduce the water concentration to arelatively low concentration as earlier described. The process may alsoinclude one or more steps of storing the TESM and/or one or more of thematerials used for preparing the TESM in a relatively dry environment(e.g., in a dessicant containing container, in a glove box with a dryatmosphere, in a hermetically sealed container, under vacuum, and thelike), such that the water concentration is maintained at a relativelylow concentration (e.g., such that the material continues to besubstantially free of water).

Other processes can be gleaned elsewhere from the teachings herein, aswell as U.S. Provisional Patent Application Ser. Nos. 61/030,755 (filedFeb. 22, 2008); 61/061,908 (filed Jun. 16, 2008); 61/074,799 (filed Jun.23, 2008); 61/074,840 (filed Jun. 23, 2008); 61/074,869 (filed Jun. 23,2008); 61/074,889 (filed Jun. 23, 2008); and 61/090,084 (filed Aug. 19,2008), all incorporated by reference.

As will be appreciated from the foregoing, though preferred materialsystems are described, the teachings herein do not foreclose others thatsatisfy the desired characteristics for a TESM. One aspect of thepresent teachings contemplates a process for designing a suitablematerial system based upon known information about candidate materialsfor the system. To illustrate, the heat storage density, the eutectictemperature, and the heat of fusion density can be estimated formixtures of N different metal salts if the melting temperature, heat offusion, density at 25° C., molar mass and the specific heat are knownfor each component in the mixture. From that information, othercharacteristics may be employed to arrive at the desired system.

Without being bound by theory, a description of the thermodynamics ofthese mixtures is presented. The entropy of fusion, S_(fi), for a singlecomponent i is given by:

S _(fi) =H _(fi) /T _(mi),

where H_(fi) is the heat of fusion of component i and T_(mi), is themelting temperature in K for component i.

For an ideal eutectic system, where the components are immiscible in thesolid state and completely miscible in the liquid state, the molar freeenergy of fusion G_(f)(T,x_(i)) is given by:

G _(f)(T,x _(i))=Σx _(i) H _(fi) m−T[(Σx _(i) S _(fi))+(Σx _(i) Inx_(i))]

where the summation is over the N components, T is the temperature in K,x_(i) is the molar fraction of component i, and the term Σx_(i) In x_(i)corresponds to the entropy of mixing of the N components.

Again without being bound by theory, by finding a solution ofG_(f)(T,x_(i))=0, one can estimate the eutectic temperature and theeutectic composition. Having thus estimated the eutectic composition,the molar heat of fusion at the eutectic composition can be calculatedfrom the equation below:

H _(f)(T _(e))=T _(e) Σx _(i) H _(fi) /T _(mi)

The heat capacity of the mixture can be estimated by the weightedaverage of the heat capacity of the individual components. The sensibleheat density (SHD_(T1,T2)) between T₁ and T₂ is calculated byintegrating the heat capacity over that temperature range:

SHD _(T1,T2) =∫ρc _(p) dT,

where ρ is the density in units of kg/l and c_(p) is the temperaturedependent heat capacity in units of MJ/(kg·° K), and the integration isfrom T₁ to T₂.

TESM systems herein may be encapsulated in a capsular structure (e.g. astructure including one or a plurality of capsules each having a volumeof less than about 200 ml, preferably less than about 50 ml, morepreferably less than about 10 ml, and most preferably less than about 3ml, and/or having a volume greater than about 0.1 ml, and morepreferably greater than about 0.3 ml). The capsular structure in turnmay be incorporated as part of an array (as described elsewhere herein),and more preferably a 3-dimensional array, where plural individualcapsules are in thermal conducting relationship with each other, andwith a heat transfer fluid. For example, the capsular structure mayinclude a plurality of stacked embossed metal foil (or other metalsheet) blister packs, which may be contained in a suitable vessel (e.g.,an insulated container, such as a vacuum insulted container) thataffords a flow path for one or more heat exchange devices and/or fluidsthat will operate to charge or discharge the TESMs.

Examples of structures suitable for encapsulating the TESMs aredisclosed in application Ser. No. ______ entitled “HEAT STORAGE DEVICES”filed on Feb. 20, 2009 (attorney docket number 67170B (1062-091)), (see,e.g., FIGS. 1-6 and 8-9 and associated discussion), which are herebyexpressly incorporated by reference herein for all purposes. As alsotaught in that application, assemblies of capsular structures (e.g., aplurality of arrays of capsules) may be assembled within a suitablehousing to define a heating module, which also may include means forcirculating a heat exchange fluid through the module (e.g., one or moreof conduits, pumps, blowers or the like). The material that is used toform the arrays or capsules preferably is a metal. Though a metal suchas aluminum may be used, a preferred metal is a stainless steel (e.g., aferritic stainless steel, an austenitic stainless steel, and/or amartensitic stainless steel). The metal preferably is such that it has ametal oxide (e.g., a native metal oxide) that exists during serviceoperation, such as on an interior wall of a capsule or other containerthat directly interfaces with the TESM. Preferably, the TESM will bestable relative to any such oxide, and will exhibit substantially nothermodynamic reactivity with it under service conditions.

The present teachings also contemplate heat storage devices, modules andsystems that employs a TESM. In one broad sense, the present teachingscontemplate the use of a TESM in a heat storage device, module or systemthat includes a housing (e.g., an insulated container or other suitablehousing), a heat source and/or heat collector for heating the TESM sothat the TESM undergoes a solid to liquid phase transition, and asuitable structure or mechanism that transfers heat from the housingsuch that the device, module, or system provides heat derived from alater liquid to solid phase transition.

The TESMs herein preferably will be such that they are used inapplications in which the operating temperatures will cause the TESMs toundergo at least one phase transition. The performance of the TESMsherein makes the TESMs particularly suitable for use in relativelycompact structures, thereby also making it attractive for use in any ofa number of methods or applications to provide custom ambient heatingsolutions, rapid ambient heating solutions, efficient heating solutions,heating solutions that allow for re-allocation of energy resources,heating solutions that allow for reduced consumption of energyresources, or any combination thereof. Systems using the TESMs hereinmay be used for delivering of heat to passengers and/or components(e.g., an engine and/or a catalyst) of a transportation vehicle (e.g.,an automotive vehicle). TESMs according to the present teachings may beemployed in internal combustion engine powered vehicles (e.g.,conventional gasoline or diesel fueled vehicles), fully electric poweredvehicles (e.g., battery powered vehicles), fuel cell powered vehicles(e.g., from the use of hydrogen fuel cells), hybrid powered vehiclesthat include a combination of two or more of such power sources. Theymay be used in combination with another heating source, for reducingload on the other heating source. It may be used in a vehicle forincreasing the range of the vehicle usage per unit of energy needed tooperate the vehicle. They may be used in a system for conversion intoelectricity via a thermoelectric device. It will be appreciated howeverthat the invention is not limited only to vehicle applications but maybe employed in other applications as well. Systems using the TESMsherein may be such that they will release greater than about 2000 kJ,and preferably greater than about 3000 kJ and most preferably greaterthan about 4000 kJ (e.g. more than about 6,000 kJ) when the thermalenergy storage material is cooled over a span of about 200° C. (e.g.,from 300° C. to 80° C.).

It will also be appreciated from the discussion herein that TESMs may beemployed in a process of reclaiming heat which includes the steps of: a)transferring at least a portion of a source heat from a heat sourceand/or a heat collector; b) heating the TESM using the source heat; c)increasing the amount of the liquid phase in the TESM by converting atleast a portion of the source heat into latent heat; d) maintaining theamount of the liquid phase in the TESM to store the latent heat; e)converting at least a portion of the latent heat into released heat; andf) transferring the released heat to an object to be heated. TESMs maybe employed in a process for reclaiming heat that may include one orboth of a) heating a combustion engine or a circulating fluid forheating a combustion engine from a temperature of less than 5° C. to atemperature of at least 60° C. in a time of less than 60 seconds, or b)heating an air stream for heating a cockpit from a temperature of lessthan 5° C. to a temperature of at least 40° C. in a time of less than 60seconds. TESMs may be employed in a process for cold starting anautomotive vehicle having a powertrain which includes the steps of: i)storing heat in a thermal energy storage material (TESM); and ii)discharging the stored heater from the TESM greater than about 2000 kJ(e.g., greater than about 3000 kJ or greater than about 4000 kJ) over aperiod less than 120 seconds while the vehicle powertrain issubstantially at ambient. The invention thus also contemplates a step ofusing the TESMs of the present teachings for heating input air fromabout 0° C. to about 30° C. at a rate of about 12 liters/second for atime of at least about 30 minutes, more preferably at least 60 minutes,and still more preferably at least 120 minutes, without applying anyexternal heat supply to any such module (e.g., without supplying anelectric current to any electric heater of the modules).

For example, various aspects of the invention may be used in heatingapplications such as vehicle applications such as i) cold start heatingof an engine (e.g., using a heat transfer fluid), a passengercompartment, a window, or any combination thereof; ii) heating of apassenger compartment, a window, or both on a plug-in electric vehicle,a plug-in hybrid electric vehicle (i.e., a PHEV), or a hybrid vehicle(HEV) using electricity from an electric grid; iii) providing multipleheating locations for more efficient and targeted heating (localized orsatellite heating); iv) heating an engine block, engine oil, a passengercompartment, or any combination thereof while the vehicle is in anengine-off mode; v) heating a battery in an electric vehicle, a PHEV, oran HEV; vi) heating a transmission or transmission oil; vii) heating awiper fluid or some other working fluid; viii) heating a catalyst; orany combination thereof. The invention may be used for heatingcomponents such as an internal combustion engine, a transmission, acatalytic converter, a cockpit (e.g., an air stream for heating acockpit), a passenger seat, a window or windshield, or a circulatingfluid (e.g., a liquid or gas) for providing heat to any of the abovecomponents. The invention may be used to heat one, two, three, four orany combination of these vehicle components. The invention may be usedfor storing heat generated by a vehicle component such as from an engineblock, a fluid circulation system, an intercooler, a radiator (e.g., airor liquid), a turbocharger, a compressor for an air conditioning unit,engine oil, a transmission, transmission fluid, an exhaust manifold, anexhaust pipe, a catalytic converter, an exhaust tip, a heat shield,mounting hardware, a muffler, a braking component, a shock absorber, oran electric resistance heater. In one preferred aspect, the systemsherein may be used in a transportation vehicle application that includesa step of i) heating an internal combustion engine or a circulatingfluid for heating a combustion engine from a temperature of less than 5°C. to a temperature of at least 60° C. in a time of less than 60seconds; ii) heating an air stream for heating a cockpit from atemperature of less than 5° C. to a temperature of at least 40° C. in atime of less than 60 seconds; or both (i) and (ii).

Additional applications which may use the heat storage device disclosedherein include: heating the air in a building or water (e.g., heating inthe winter using solar energy collected in the summer); heating abuilding with a furnace having improved efficiency; heatingnon-automotive battery cells; heating an electrochemical battery; andheating of floors using a solar hydronic system, an electric system orboth. Additional examples of objects that may be heated include a fluidvessel (e.g., the water in a hot water tank), a circulating fluid (e.g.,an air stream or a liquid) for heating a building, a cooking device, aturbine, a hot plate, a laundry dryer (i.e., a tumble dryer), a heatengine (e.g., a Rankine or Brayton Cycle) to generate electric power orto power a compressor of an air conditioning unit, and a working fluidof an absorption or adsorption cycle air conditioning system. Additionalheat sources thus may include heat generated by one or more of: the sun,a lawn mower motor, a motor for a snow removal device, a conveyer (e.g.,an escalator, an elevator, or a conveyor belt), an oven, a homeappliance, paving equipment, a watercraft motor, a solar heat collector,an exhaust stack, a non-automotive braking system, a resistance heater,a chemical reactor, a condenser unit of an air conditioning system and ageothermal circulating fluid.

The heating modules may contain any amount of TESM. Preferably theplurality of heating modules contain a total volume of thermal energystorage material, V_(t) wherein V_(t) may be less than about 8 liters,preferably less than about 5 liters, and more preferably less than about3 liters. The total mass of the TESM in the plurality of heating modulesmay be less than about 18 kg, preferably less than about 10 kg, morepreferably less than about 7 kg, and most preferably less than about 5kg.

TESMs, heating devices and systems (e.g., heat storage devices and heatstorage systems), heating modulus, and heat storage process which may beuseful for the vehicle components and heating processes of the presentinvention are further described in U.S. Provisional Patent ApplicationSer. Nos. 61/030,755 (filed Feb. 22, 2008); 61/061,908 (filed Jun. 16,2008); 61/074,799 (filed Jun. 23, 2008); 61/074,840 (filed Jun. 23,2008); 61/074,869 (filed Jun. 23, 2008); 61/074,889 (filed Jun. 23,2008); and 61/090,084 (filed Aug. 19, 2008), and U.S. patent applicationSer. No. ______ entitled “HEAT STORAGE DEVICES” filed on Feb. 20, 2009(attorney docket number 67170B (1062-091)), the contents of which arehereby incorporated by reference in their entirety.

Examples

For the following examples, similar results are expected for variationsof +/−20% of the stated concentration amounts. For example, resultsdescribed in the following are expected to be within +/−30% of thestated values for concentrations that may vary by +/−20% of the statedvalues.

Example 1

A 5 g mixture containing 40 mole % of powdered lithium nitrate and 60mole % of powdered lithium hydroxide is prepared by first mixing wetgrinding and mixing the two anhydrous salts with a mortar and pestle inacetone. The acetone slurry is then poured into a fused silica cruciblewith a fused silica lid, both lined with aluminum foil. After vacuumdrying at room temperature to remove the acetone, the crucibles areplaced in a furnace and heated from room temperature to 300° C. at arate of 5° C./min. The samples are held at 300° C. for 1 hour. While atthe elevated temperature, the samples are stirred about every 5 minutesby shaking the entire furnace. These binary mixtures of lithium nitrateand lithium hydroxide are then cooled to room temperature. The liquidustemperature and the heat storage density from 300° C. to 80° C. of thesesamples is then measured. The heat storage density (HSD_(300,80)) isgreater than 1.4 MJ/l and the liquidus temperature is about 190° C.

Example 2

A 5 g mixture of 85 mole % lithium nitrate and 15 mole % of an eutecticmixture of a mixed metal fluoride salt containing MgF₂, NaF, and LiF(having a ratio of MgF₂:NaF:LiF of about 10:43:47 and a eutectictransition temperature of about 630° C.) is prepared in a manner similarto Example 1. To homogenize the metal salts, a furnace is heated to 500°C. at 5° C./min, and is then held for 1 hour at 500° C. The mixed metalfluoride dissolves in the molten lithium nitrate under these conditions.The sample is then cooled to room temperature. The sample is polished inorder to perform optical microscopy studies of the structure and forcompositional analysis using electron microscopy. The liquidustemperature and the heat storage density from 300° C. to 80° C. of thesesamples is then measured. The heat storage density (HSD_(300,80)) isgreater than 1.6 MJ/l and the liquidus temperature is about 196° C.

Example 3

A 5 g mixture of 85 mole % lithium nitrate and 15 mole % of an eutecticmixture of a mixed metal salt containing NaCl, NaF, and LiF (having aratio of NaCl:NaF:LiF of about 24:36:40 and a eutectic transitiontemperature of 582° C.) is prepared in a manner similar to Example 2.After homogenizing at 500° C. for 1 hour and cooling to room temperaturethis sample is polished in order to perform optical microscopy studiesof the structure and for compositional analysis using electronmicroscopy. The liquidus temperature and the heat storage density from300° C. to 80° C. of these samples is then measured. The heat storagedensity (HSD_(300,80)) is greater than 1.4 MJ/l and the liquidustemperature is about 250° C.

Example 4

A 5 g mixture of lithium nitrate, lithium hydroxide and an eutecticmixture of a mixed metal fluoride salt containing MgF₂, NaF, and LiF(having a ratio of MgF₂:NaF:LiF of about 10:43:47 and a eutectictransition temperature of about 630° C.) is prepared in a manner similarto Example 2. The liquidus temperature and the heat storage density from300° C. to 80° C. of these samples is then measured. The heat storagedensity (HSD_(300,80)) is greater than 1.4 MJ/l and the liquidustemperature is about 180° C.

Example 5

A thermal energy storage material, LT1, is prepared by mixing about 66.2wt % LiNO₃, about 16.5 wt % NaNO₂ and about 17.3 wt % KNO₂ using theprocess of Example 1. The liquidus temperature and the heat storagedensity from 200° C. to 50° C. of these samples is then measured. Theheat storage density (HSD_(200,50)) is greater than 1.1 MJ/l and theliquidus temperature is about 135° C.

Example 6

A thermal energy storage material, LT2, is prepared by mixing about 62.4wt % LiNO₃, and about 37.6 wt % NaNO₂ using the process of Example 1.The liquidus temperature and the heat storage density from 200° C. to50° C. of these samples is then measured. The heat storage density(HSD_(200,50)) is greater than 1.2 MJ/l and the liquidus temperature isabout 150° C.

Example 7

A thermal energy storage material, MT1, is prepared by mixing about 48wt % LiNO₃, and about 52 wt % NaNO₃ using the process of Example 1. Theliquidus temperature and the heat storage density from 300° C. to 80° C.of these samples is then measured. The heat storage density(HSD_(300,80)) is greater than about 1.5 MJ/l and the liquidustemperature is about 220° C.

Example 8

A thermal energy storage material is heated in a differential scanningcalorimeter to a temperature of about 325° C. and then cooled to about50° C. at a rate of about 10° C./min. The sensible and latent heats arerecorded as represented in FIG. 1. The heat storage density from 300° C.to 80° C. (HSD_(300,80)) is measured by integrated the area under thecurve between these temperatures. The heat storage density, thusmeasured is about 1.5 MJ/l.

Example 9

An experimental thermal energy storage material, HT (i.e., Dow HT), isprepared having a HSD_(300,80) greater than about 1.9 MJ/l and having aliquidus temperature of about 260° C. A 5 mg sample of HT is placed inan aluminum pan and covered. The covered pan is placed in a differentialscanning calorimeter (DSC). The temperature is cycled between about 320°C. and about 20° C. The heat storage density from about 320° C. to about20° C. a HSD_(320,20,i) is measured for each thermal cycle i. Initially,HT has an HSD_(320,20,0) of about 2.35 MJ/l. After 100 cycles, HT has anHSD_(320,20,100) of about 2.28 MJ/l. The decrease in the HSD_(32,20) isbelow about 3% after 100 cycles, and (as with other materials herein) isexpected to maintain approximately that level for 500, or even 1000 ormore cycles. FIG. 5A shows the DSC heating curves (heat capacity inJ/g·° C. vs. temperature in ° C.) for the first cycle (i=0), the 51 stcycle (i=50) and the 101^(st) cycle (i=100) These curves have beenvertically offset in the figure so that the peaks associated with themelting of the TESM can be easily differentiated for each heating cycle.The baseline of each curve illustrates that the heat capacity is nearlyconstant (constant rate of sensible heat increase with increasingtemperature) and the peaks illustrates the latent heat of fusion. InFIG. 5A the absolute heat capacity is not shown, since the curves areshifted. FIG. 5B illustrates the HSD_(320,20,i) of sample HT as afunction of the thermal cycle, i. FIG. 5C illustrates the cumulativeheat (combined sensible heat and latent heat of fusion) as a function oftemperature for i=0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, where theintegration begins at a temperature of 20° C. The overlay of thesecurves illustrates the good stability of this material. The thermalproperties (onset of melting in ° C., peak melting temperature in ° C.,heat of fusion in J/g, onset of crystallization in ° C., and heat ofcrystallization in J/g) of the sample after i=0, 10, 20, 30, 40, 50, 60,70, 80, 90, and 100 cycles is tabulated in TABLE 1 below.

TABLE 1 CYCLIC THERMODYNAMIC PROPERTIES OF SAMPLE HT Heat-Cool PeakMelting Heat of Cycle Onset Melting Temperature, Heat of CrystallizationCrystallization, Number Temperature, ° C. ° C. Fusion, J/g Onset Temp.,° C. J/g 1 253 256 373 * * 11 252 255 372 * * 21 252 255 368 * * 31 252255 345 250 355 41 252 255 342 249 353 51 251 255 340 249 343 61 251 254334 248 334 71 250 254 330 249 327 81 250 254 323 248 325 91 250 254 319247 321 101 249 253 317 246 312 * Crystallization was so fast thatsample self heated.

A fresh specimen of sample HT is heated in a differential scanningcalorimeter (DSC). The sample is first heated from about 20° C. to about320° C. at a rate of about 10° C./min and then cooled to 20° C. at arate of about −10° C./min. A peak melting temperature of about 260° C.and a heat of fusion of about 352 J/g is measured during the first heatcycle. An onset of crystallization of about 249° C. and a heat ofcrystallization of about 355 J/g is measured during the first coolingcycle. The DSC heating and cooling curves for the first cycles areillustrated in FIG. 6A. The change in mass of the sample is about 2%.For the second cycle, the sample is heated to about 500° C. at a rate of10° C./min and then cooled to 20° C. at a rate of about −10° C./min. Thesecond cycle is characterized by a heat of fusion of about 346 J/g, apeak melting temperature of about 259° C., an onset of crystallizationof about 242° C. and a heat of crystallization of about 314 J/g asillustrated in the top curves of FIG. 6B. The change in mass of thesample during the second cycle is about 2%. The specimen is thenreheated in a third cycle to 500° C. and the heat of fusion is about 304J/g and the peak melting temperature is about 255° C. as illustrated inthe bottom curve of FIG. 6B.

Example 10

An experimental thermal energy storage material, MT1 (i.e., Dow MT1), isprepared having a HSD_(300,80)) greater than about 1.7 MJ/l and having aliquidus temperature of about 210° C. A 5 mg sample of MT1 is placed inan aluminum pan and covered. The covered pan is placed in a differentialscanning calorimeter. The temperature is cycled between about 305° C.and about 40° C. The heat storage density from about 305° C. to about40° C. a HSD_(305,40,i) is measured for each thermal cycle i. Initially,MT1 has an HSD_(305,40,0) of about 1.94 MJ/l. After 100 cycles, MT1 hasan HSD_(305,40,100) of about 1.85 MJ/l. The decrease in theHSD_(305,40,100) is below about 5% of HSD_(305,40,0). FIG. 7A shows theDSC heating curves (heat capacity in J/g·° C. vs. temperature in ° C.)for the first cycle (i=0), the 51^(st) cycle (i=50) and the 101^(st)cycle (i=100). These curves have been vertically offset in the figure sothat the peaks associated with the melting of the TESM can be easilydifferentiated for each heating cycle. The baseline of each curveillustrates that the heat capacity is nearly constant (constant rate ofsensible heat increase with increasing temperature) and the peaksillustrates the latent heat of fusion. In FIG. 7A the absolute heatcapacity is not shown, since the curves are shifted. FIG. 7B illustratesthe HSD_(305,40,i) (in units of MJ/liter) of sample MT1 as a function ofthe thermal cycle, i.

FIG. 7C illustrates the cumulative heat (combined sensible heat andlatent heat of fusion) as a function of temperature for i=0, 10, 20, 30,40, 50, 60, 70, 80, 90, 100, where the integration begins at atemperature of 40° C. The thermal properties (onset of melting in ° C.,peak melting temperature in ° C., heat of fusion in J/g, onset ofcrystallization in ° C., and heat of crystallization in J/g) of thesample after i=0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 cycles istabulated in TABLE 2 below.

TABLE 2 CYCLIC THERMODYNAMIC PROPERTIES OF SAMPLE MT1 Heat-Cool PeakMelting Heat of Cycle Onset Melting Temperature, Heat of CrystallizationCrystallization, Number Temperature, ° C. ° C. Fusion, J/g Onset Temp.,° C. J/g 1 200 202 284 204 259 11 194 196 267 197 241 21 194 196 264 206182 31 194 196 263 193 135 41 194 196 262 205 208 51 194 196 261 198 21161 194 196 260 196 235 71 194 196 258 196 236 81 194 196 258 197 234 91194 196 256 196 234 101 194 196 255 197 231

A fresh specimen of sample MT1 is heated in a differential scanningcalorimeter (DSC). The sample is first heated from about 20° C. to about320° C. at a rate of about 10° C./min and then cooled to 20° C. at arate of about −10° C./min. A peak melting temperature of about 205° C.and a heat of fusion of about 259 J/g is measured during the first heatcycle. An onset of crystallization of about 188° C. and a heat ofcrystallization of about 239 J/g is measured during the first coolingcycle. The DSC heating and cooling curves for the first cycles areillustrated in FIG. 8A. The change in mass of the sample is about 0%.For the second cycle, the sample is heated to about 500° C. at a rate of10° C./min and then cooled to 20° C. at a rate of about −10° C./min. Thesecond cycle is characterized by a heat of fusion of about 258 J/g, apeak melting temperature of about 202° C., an onset of crystallizationof about 188° C. and a heat of crystallization of about 228 J/g asillustrated in the top curves of FIG. 8B. The change in mass of thesample during the second cycle is about 4.6%. The specimen is thenreheated in a third cycle to 500° C. and the heat of fusion is about 236J/g and the peak melting temperature is about 198° C. as illustrated inthe bottom curve of FIG. 8B.

Example 11

An experimental thermal energy storage material, LT (i.e., Dow LT), isprepared having a HSD_(200,50)) greater than about 0.90 MJ/l and havinga liquidus temperature of about 125° C. A 5 mg sample of LT is placed inan aluminum pan and covered. The covered pan is placed in a differentialscanning calorimeter. The temperature is cycled between about 200° C.and about 25° C. The heat storage density from about 200° C. to about25° C. a HSD_(200,25,i) is measured for each thermal cycle i. Initially,LT has an HSD_(200,50,0) of about 1.7 MJ/l. After 100 cycles, LT has anHSD_(200,25,100) of about 15 MJ/l. The decrease in the HSD_(200,25,100)is at most about 18% of HSD_(305,40,0). FIG. 9A shows the DSC heatingcurves (heat capacity in J/g·° C. vs. temperature in ° C.) for the firstcycle (i=0), the 51^(st) cycle (i=50) and the 101^(st) cycle (i=100).These curves have been vertically offset in the figure so that the peaksassociated with the melting of the TESM can be easily differentiated foreach heating cycle. The baseline of each curve illustrates that the heatcapacity is nearly constant (constant rate of sensible heat increasewith increasing temperature) and the peaks illustrates the latent heatof fusion. In FIG. 9A the absolute heat capacity is not shown, since thecurves are shifted. FIG. 9B illustrates the HSD_(200,25,i) (in units ofMJ/liter) of LT as a function of the thermal cycle, i. FIG. 9C thecumulative heat (combined sensible heat and latent heat of fusion) as afunction of temperature for i=0, 10, 20, 30, 40, 50, 60, 70, 80, 90,100, where the integration begins at a temperature of 25° C. The thermalproperties (onset of melting in ° C., peak melting temperature in ° C.,heat of fusion in J/g, onset of crystallization in ° C., and heat ofcrystallization in J/g) of the sample after i=0, 10, 20, 30, 40, 50, 60,70, 80, 90, and 100 cycles is tabulated in TABLE 3 below.

TABLE 3 CYCLIC THERMODYNAMIC PROPERTIES OF SAMPLE LT Heat-Cool PeakMelting Heat of Cycle Onset Melting Temperature, Heat of CrystallizationCrystallization, Number Temperature, ° C. ° C. Fusion, J/g Onset Temp.,° C. J/g 1 41 104 182 52 45 11 97 125 176 47 55 21 97 125 171 38 5 31 95125 172 42 8 41 96 125 168 29 7 51 97 125 148 38 5 61 93 125 168 39 4 7193 125 166 32 1 81 92 125 166 39 3 91 92 125 164 43 4 101 94 125 150 5932

Example 12

A sample of a TESM of the present invention (EX. 1) and sample of acomparative TESM (Comp. Ex. 1) are studied by differential scanningcalorimetry. Each sample is heated above its liquidus temperature at arate of 10° C./min and then cooled at a rate of −10° C./min. The heatingand cooling curves are illustrated in FIG. 10. The Comp. EX. 1 samplecontains erythritol which is an organic TESM. The comparative example 1has a high hysteresis and only releases about half of its stored thermalenergy upon cooling. The sample designated as EX. 1 is a TESM of thepresent invention and has a low hysteresis with most of the stored heatrecovered upon cooling. The melting and crystallization temperatures ofEx. 1 differ by less than about 20° C.

Example 13

Erythritol Comparison: By way of comparison, FIG. 11 further illustratesthe heating and cooling behavior of erythritol. During the first heatcycle, the erythritol melts. Upon cooling at 10° C., there is little orno latent heat of fusion, and the majority (or all) of the heatrecovered is sensible heat. The majority of the crystallization occursdoes not occur until the second heating cycle, when the materialcrystallizes as the sample is heated and releases heat. Again, duringthe second cooling cycle, there is little or no latent heat of fusion,and the majority (or all) of the heat recovered is sensible heat. In thethird heating cycle, crystallization is not observed. The heating andcooling curves only show sensible heat.

Table 4 below illustrates selected physical and thermal characteristicsof various TESMs including a high temperature TESM which has a phasetransition at a temperature below about 300° C.; a medium temperatureTESM which has a phase transition at a temperature below about 250° C.;and a low temperature TESM which may has a phase transition at atemperature below about 200° C. Table 4 also lists properties of anorganic TESM, erythritol, and a hydrated salt, barium hydroxideoctahydrate.

TABLE 4 THERMODYNAMIC PROPERTIES OF THERMAL ENERGY STORAGE MATERIALSMaterial Dow HT Dow MT1 (High (Medium Dow LT Barium Hydroxide Temp)Temp) (Low Temp) Erythritol Octahydrate Composition LiNO₃ LiNO₃*Cations: Li, Na, K C₄H₁₀O₄ Ba(OH)₂•8H₂0 NaNO₃ Anions: NO₃, NO₂ Density(g/cm3) 2.38 2.32 2.27 1.45 2.18 Weight of 1.65 3.9 3.8 3.7 2.4 3.6liters (kg) Temperature 50-300 50-250 50-200 50-150 50-105 range, ° C.Sensible Heat 1.10 0.5 0.63 0.29 0.23 (MJ/L) Center of Melting 255 200120 125 80 Range, ° C. Latent Heat, 0.92 0.61 0.48 0.53 0.65 MJ/L Dow LTis a mixture of LiNO₃, NaNO₂, and KNO₂ and crystallized from the melt.

Examples 14-43 Examples 14-43 Illustrate Additional TESM Compositions

14 15 16 17 18 19 20 21 22 23 LiNO₃, mole % 83.7% 79.8% 71.1% 59.0%60.2% 64.5% 80.6% 68.4% 100.0% 60.8% LiF, mole % 0.0% 0.0% 28.9% 10.9%19.7% 7.5% 13.7% 10.6% 0.0% 24.0% NaF, mole % 11.7% 20.2% 0.0% 20.1%9.4% 23.6% 0.0% 10.3% 0.0% 15.2% MgF₂, mole % 4.5% 0.0% 0.0% 10.0% 10.7%4.5% 5.8% 10.7% 0.0% 0.0% Density, g/cm³ 2.42 2.4 2.41 2.48 2.48 2.442.43 2.47 2.38 2.43 Cumulative 744 743 768 622 680 639 742 672 799 637Heat (300 to 80° C.), J/g HSD_(300,80), MJ/L 1.8 1.78 1.85 1.54 1.691.56 1.8 1.66 1.9 1.55 Melt Temp(s), 196 197 256 199 192 199 255 196 260198 ° C. 238 228 244 253 236 223 253 Heat(s) of 86.5 144.6 314.8 193.833.1 207.7 321 80.7 350.7 127.5 Fusion, J/g 200.7 134.6 220.0 2.15 173.283.3 8.4 24 25 26 27 28 29 30 31 32 33 LiNO₃, mole % 63.1% 75.4% 85.4%71.9% 62.9% 79.9% 58.6% 95.2% 70.5% 69.0% LiF, mole % 32.4% 0.0% 14.6%14.8% 29.1% 0.0% 31.0% 0.0% 10.7% 0.0% NaF, mole % 4.5% 19.6% 0.0% 0.0%3.9% 10.4% 0.0% 0.0% 18.8% 20.9% MgF₂, mole % 0.0% 4.9% 0.0% 13.3% 4.1%9.7% 10.4% 4.8% 0.0% 10.2% Density, 2.42 2.43 2.39 2.48 2.45 2.45 2.492.41 2.41 2.46 g/cm³ Cumulative 720 682 775 709 705 729 670 749 698 647Heat (300 to 80° C.) J/g HSD_(300,80), 1.74 1.66 1.85 1.76 1.73 1.781.67 1.8 1.68 1.59 MJ/L Melt Temp(s), 193 199 254 253 195 196 255 260198 201 253 ° C. 245 222 247 243 221 253 253 253 254 Heat(s) of 31.5 157323 282 26.8 57.7 253.8 337.1 161.4 219.6 Fusion, J/g 228.7 80 234 225.580.6 2.5 9.0 4 6.9 6.5 34 35 36 37 38 39 40 41 42 43 LiNO₃, mole % 89.2%58.9% 59.7% 72.7% 59.8% 64.2% 60.5% 90.2% 60.1% 64.7% LiF, mole % 0.0%29.8% 20.1% 16.1% 14.3% 30.5% 30.0% 0.0% 22.7% 4.6% NaF, mole % 10.8%5.0% 20.2% 11.2% 20.8% 0.0% 9.4% 0.0% 12.6% 19.4% MgF₂, mole % 0.0% 6.3%0.0% 0.0% 5.1% 5.3% 0.0% 9.8% 4.6% 11.3% Density, 2.39 2.46 2.43 2.412.46 2.45 2.43 2.43 2.45 2.48 g/cm³ Cumulative 750 687 618 717 615 717652 753 671 639 Heat (300 to 80° C.), J/g HSD_(300,80), 1.79 1.69 1.51.73 1.51 1.76 1.58 1.83 1.65 1.58 MJ/L Melt Temp(s), 196 195 199 195194 256 196 262 197 198 ° C. 239 238 253 238 234 228 253 254 254 254 254254 Heat(s) of 76.5 39.3 197.3 74 199 290 85 320 108 212 Fusion, J/g217.9 198.9 4.2 193 154 114 5 8.0 8.9 5 4 6

Examples 44-46

A sample of metal foil having a thickness of about 20 to 100 μm and atotal surface area of about 60 cm² is weighed and then placed in acrucible made of the same material as the metal foil. The crucible isfilled with the heat storage material of Example 7 (MT1) such that theTESM contacts the entire surface of the metal foil. The crucible issealed and placed in an autoclave at about 300° C. for 45 days. After 45days, the crucible is cooled to room temperature and the weight changeof the metal foil is determined. The test is repeated using twospecimens of each metal. Example 44 is an aluminum foil (Al 1100) havingan initial thickness of about 75 μm thick. After 45 days in theautoclave the aluminum has an average weight gain of about 8.7 mg.Example 45 is a stainless steel (304 type) having an initial thicknessof about 125 μm. After 45 days in the autoclave the 304 stainless steelhas an average weight gain of about 0.55 mg. Example 46 is a stainlesssteel (316 type) having an initial thickness of about 125 μm. After 45days in the autoclave the 316 stainless steel has an average weight gainof about 0.45 mg.

Sample Foil Duration Initial Final Weight Name Material (Days) Weight(g) Weight (g) Gain (mg) EX. 44 AI1100 45 0.6159 0.6253 9.4 EX. 44AI1100 45 0.5710 0.5790 8.0 EX. 45 SS304 45 2.9059 2.9064 0.5 EX. 45SS304 45 2.8621 2.8627 0.6 EX. 46 SS316 45 3.0077 3.0081 0.4 EX. 46SS316 45 2.9542 2.9547 0.5

The following discussion applies to the teachings as a whole, and notmerely the teachings of the examples. Unless otherwise stated, allranges include both endpoints and all numbers between the endpoints. Theuse of “about” or “approximately” in connection with a range applies toboth ends of the range. Thus, “about 20 to 30” is intended to cover“about 20 to about 30”, inclusive of at least the specified endpoints.Unless otherwise stated, references to containers contemplate interimstorage containers as well as permanent encapsulation structures.

The disclosures of all articles and references, including patentapplications and publications, are incorporated by reference for allpurposes. References to an The term “consisting essentially of” todescribe a combination shall include the elements, ingredients,components or steps identified, and such other elements ingredients,components or steps that do not materially affect the basic and novelcharacteristics of the combination. The use of the terms “comprising” or“including” to describe combinations of elements, ingredients,components or steps herein also contemplates embodiments that consistessentially of the elements, ingredients, components or steps.

Plural elements, ingredients, components or steps can be provided by asingle integrated element, ingredient, component or step. Alternatively,a single integrated element, ingredient, component or step might bedivided into separate plural elements, ingredients, components or steps.The disclosure of “a” or “one” to describe an element, ingredient,component or step is not intended to foreclose additional elements,ingredients, components or steps. Likewise, any reference to “first” or“second” items is not intended to foreclose additional items (e.g.,third, fourth, or more items), or to indicate a particular preference ororder; such additional items are also contemplated, unless otherwisestated. All references herein to elements or metals belonging to acertain Group refer to the Periodic Table of the Elements published andcopyrighted by CRC Press, Inc., 1989. Any reference to the Group orGroups shall be to the Group or Groups as reflected in this PeriodicTable of the Elements using the IUPAC system for numbering groups.

It is understood that the above description is intended to beillustrative and not restrictive. Many embodiments as well as manyapplications besides the examples provided will be apparent to those ofskill in the art upon reading the above description. It is furtherintended that any combination of the features of different aspects orembodiments of the invention may be combined. The scope of the inventionshould, therefore, be determined not with reference to the abovedescription, but should instead be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled. The disclosures of all articles and references,including patent applications and publications, are incorporated byreference for all purposes. The omission in the following claims of anyaspect of subject matter that is disclosed herein is not a disclaimer ofsuch subject matter, nor should it be regarded that the inventors didnot consider such subject matter to be part of the disclosed inventivesubject matter.

1. A thermal energy storage material (TESM) system that reproduciblystores and recovers latent heat comprising: a. a container having a wallsurface; and b. a TESM in at least partial contact with the wallsurface, and including: i) at least one first metal containing materialincluding at least one first metal compound that includes a nitrate ion,a nitrite ion, or both; ii) at least one second metal containingmaterial including at least one second metal compound; and iii)optionally including water, wherein the water concentration if any ispresent is less than about 10 wt. %; wherein the TESM has a liquidustemperature, T_(L), from about 100° C. to about 250° C.; and wherein theTESM exhibits a heat storage density from 300° C. to 80° C. of at leastabout 1 MJ/l; so that upon being used in a system that generates heat,at least a portion of the heat is captured and stored by the TESM andsubsequently released for use, and wherein the absolute value of thechange in mass of the wall surface in contact with the TESM is less thanabout 1 g per m² of the wall surface in contact with the TESM after 45days exposure to the TESM at 300° C. in an inert atmosphere.
 2. The TESMsystem of claim 1 wherein the TESM is enclosed in the container whereinthe container has a cavity and the wall surface includes a metal oxide.3. The TESM system of claim 1 wherein the concentration of hydrogenatoms in the TESM, if any is present is less than 5 mole %, based on thetotal concentration of atoms in the TESM.
 4. The TESM system of claim 1wherein the TESM has a value of H_(C50)/H_(C1) greater than 70%, whereH_(C50) is defined as the quenched heat of crystallization measured bydifferential scanning calorimetry at a cooling rate of about 50° C./minover a temperature range of T_(L)+50° C. to T_(L)−100° C. and H_(C1) isdefined as the slow cooled heat of crystallization measured bydifferential scanning calorimetry at a cooling rate of about 1° C./min.5. The TESM system of claim 1 wherein the TESM includes lithium nitrateand sodium nitrate.
 6. The TESM system of claim 1 wherein the TESMincludes lithium nitrate at a concentration from about 35 mole % toabout 65 mole % based on the total moles of thermal energy storagematerial.
 7. The TESM system claim 1 wherein the TESM includes sodiumnitrate at a concentration from about 35 mole % to about 65 mole % basedon the total moles of thermal energy storage material.
 8. The TESMsystem of claim 1 wherein the TESM includes a nitrate ion and a nitriteion.
 9. The TESM system of claim 1 wherein the TESM optionally includeswater, wherein the water concentration if any is present is less thanabout 2 wt. % based on the total weight of the TESM.
 10. The TESM systemof claim 1 wherein the TESM includes a nucleator.
 11. The TESM system ofclaim 1 wherein the TESM fills 70% or more of the cavity of thecontainer.
 12. The TESM system of claim 1, wherein the TESM includes ananti-corrosion agent.
 13. The TESM system of claim 1, wherein the TESMcontains lithium cations at a concentration from about 40 mole % toabout 80 mole % based on the total concentration of cations in the TESM.14. The TESM system of claim 1 wherein the TESM has a waterconcentration, if any is present, less than about 2 wt. %, based on thetotal concentration of the TESM.
 15. The TESM system of claim 1 whereinthe TESM has a liquidus temperature which is less than T_(min) −25° C.,wherein T_(min) is the lowest melting temperature of any binary saltwhich can be formed by any anion of the TESM and any cation of the TESM.16. The TESM system of claim 1 wherein the total concentration oflithium nitrate and sodium nitrate is greater than about 95 wt. % basedon the total weight of the TESM.
 17. The TESM system claim 1, whereinthe TESM is a ternary material having a eutectic composition.
 18. Amethod of making a TESM system of claim 1, comprising encapsulating theTESM in a volume that is substantially free of water.
 19. The use of aTESM system of claim 1 to store and discharge heat.
 20. A thermal energystorage material (TESM) system that reproducibly stores and recoverslatent heat comprising: a. a container having a wall surface; and b. aTESM in at least partial contact with the wall surface, and including aeutectic composition of i) a lithium salt selected from the groupconsisting of lithium nitrate and lithium nitrite; ii) a sodium saltselected from the group consisting of sodium nitrate and sodium nitrite;wherein the TESM optionally including water, wherein the waterconcentration if any is present is less than about 2 wt. % based on thetotal weight of the TESM; wherein the TESM has a liquidus temperature,T_(L), from about 100° C. to about 250° C.; wherein the TESM exhibits aheat storage density from 300° C. to 80° C. of at least about 1 MJ/l; sothat upon being used in a system that generates heat, at least a portionof the heat is captured and stored by the TESM and subsequently releasedfor use, and wherein the absolute value of the change in mass of thewall surface in contact with the TESM is less than about 1 g per m² ofthe wall surface in contact with the TESM after 45 days exposure to theTESM at 300° C. in an inert atmosphere.